This invention relates generally to methods and apparatus for determining the density and flow rates of fluid in a well or pipe and, more specifically, to methods and apparatus for determining the average density and individual flow rates of the different components of a fluid flow regime consisting of more than one fluid phase.
In producing wells it is common to find the well fluid consisting of multiple phases, such as oil and water, oil and gas, or oil, water, and gas. Often, one or more of these phases is an undesirable segment of the well production. For example, in the case of a flow regime consisting of oil and water, as is common in such producing wells, the oil is the fluid phase desired to be produced while the water phase is typically an undesired element in the production flow. When the degree of water present in the production flow becomes excessive, logging surveys are run to determine the locations and rates of the water entry into the flow regime. These surveys include measurements of the flow rate and attempts at determining the average density of the well fluid at a survey depth. Using these measurements and data derived at the earth's surface of the densities of the individual phases, determinations are made of the volumetric fractions of water and oil in the flow, such fractions aiding the determination of the locations and rates of water entry. From these determinations remedial actions to control the water entry may be chosen.
Measurement of the flow rate and density of the well fluid is complicated by the fact that not only do the individual phases of the flow regime flow at different velocities, referred to as phase slippage, but also the nature of the flow pattern is not uniform throughout cross-sections of the pipe. This non-uniformity of the flow pattern is caused by one or more of a multiplicity of phenomena which are known in the art, such as, for example, stagnation, heavy-phase fall-back, and circulation, and is accentuated by such factors as large pipe, low flow rates, and/or deviated boreholes.
One means known to the oil and gas industry to measure the flow rate in a producing well is to use a packer-type flowmeter. These flowmeters consist of a packer mechanism which is lowered in the borehole to the desired survey depth and locked into position, blocking the fluid flow around the device and causing the fluid to flow through an aperture in the packer which contains, or is coupled to, a means for measuring the rate of the fluid flow. These packer-type flowmeters are often difficult to properly secure in position and to remove after the measurement has been taken and typically may not be used in wells having relatively high flow rates because the pressure of the fluid flow will cause the packer to unseat from its established position. Further, where the fluid flow regime consists of two or more phases traveling at different velocities, it is believed that the relative composition of the fluid flow regime, and therefore the overall measured flow rate, is altered by the abrupt blockage of the fluid flow path by the packer.
Another means by which the oil and gas industry has attempted to determine both flow rates and densities has been to intersect the fluid flow regime with the appropriate logging instrument while allowing the fluid flow to continue around the instrument. This type of measurement has been taken with the logging instrument either centralized within the borehole or allowed to decentralize and rest along the lower side of the borehole. It can be appreciated that this type of measuring only determines the flow rate or density of such portion of the fluid flow regime as actually engages the measuring system of the logging instrument. Therefore, fluid phases which do not intersect the instrument or non-uniformities in the flow pattern, caused by effects such as those described above, which occur in the flow regime may cause the readings from the logging instrument to yield less than optimal data as to the nature of the fluid flow regime. Additionally, the accuracy of this type of surveying may be further complicated by the unknown effects upon the multi-phase flow regime when a logging tool is introduced into the producing well.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing an apparatus and method whereby the average density of a multi-phase flow regime may be determined from a single measurement while minimizing phenomena which would disturb the accuracy of the measurement, and, whereby the volumetric fractions and the flow rates of the individual phases of the flow regime may be more accurately determined in accordance with the density measurement thus obtained.